The detection of the accumulation of an undesirable layer on a surface, particularly in the case of wing icing, is an area of interest. The reliable detection of this buildup as well as the ability to discriminate between various forms of accumulation are important objectives for such a system.
Very little ice is needed to impair the flying ability of aircraft, particularly during low speed flight. Many detection schemes have been proposed for determining the presence of a layer of ice on wing airfoil surfaces but reliable detection of this situation, without having devices mounted externally on the wing, is difficult. The presence of external instrumentation which would facilitate detection is undesirable from the standpoint of maintaining a smooth airflow across the wing during flight. This essentially restricts the placement of instrumentation, if large portions of the wing are to be monitored, to structures inside the wing. Even there, the addition of large amounts of weight to the wing structure are to be avoided particularly at remote wing points where leverage augments the effect of such weights. Furthermore, the addition of electrical circuits and heaters in the vicinity of fuel may not be too desirable in some applications.
If sound waves are applied to the wing airfoil plate and then detected, either at some remote point or after reflection of the sound energy from reflecting barriers in the airfoil plate, two problems are immediately encountered in attempting to detect ice. In the first, sonic wave used for detection purposes may be submerged in sound energy generated by aircraft machinery, such as engines, compressors, and other devices normally encountered on a large passenger airplane. In addition, the compressional component of the sonic wave is affected relatively little by even significant accumulations of ice.
Precipitation can accumulate on an aircraft wing in a variety of different forms which include a layer of water, a layer of frost, a layer of slush, a layer of snow, a layer of ice of variable thickness which may, or may not, be frozen to the wing plate. Flight decisions made by the pilot or others may depend upon the detection of only only the presence of an accumulation, but its nature, location and the degree to which it is attached to the wing surface. Furthermore, the cost of de-icing airplanes before take-off may be reduced by minimizing the number of deicings by continuously monitoring the aircraft condition.